Conventionally, a television switch module is known which switches and outputs multiple line input signals of e.g. CATV (Cable Television) and terrestrial (Air) TV to specified output terminals (refer to e.g. Japanese Laid-open Patent Publication Hei 7-15715). Generally, this kind of television switch module has a built-in amplifier for amplifying signals, but the above-described patent publication does not describe power control of the amplifier.
A television switch module with a built-in amplifier of this kind is shown in FIG. 11, and will be described hereinafter. This television switch module comprises: two input lines 11, 21 (INPUT 1, INPUT 2) to which television high frequency signals are respectively input; amplifiers 12, 22 (AMP 1, AMP 2) for amplifying input signals; branching units 13, 23 for branching the input signals; two relay switches 16, 26 (SW 1, SW 2) subsequently connected for optionally selecting and switching signal output lines; two output lines 17, 27 (OUTPUT 1, OUTPUT 2); and a power supply/control interface 30 for operation to switch the relay switches 16, 26 and for providing power supply to the amplifiers 12, 22. The relay switches 16, 26 have relay switch contacts and a relay common contact to which lines 14, 15 and 24, 25 following the branching by the branching units 13, 23 are respectively connected. The common contacts are connected to the respective output lines 17, 27. Reference numeral 31 is a relay control line, and reference numeral 32 is a power supply line to the amplifiers 12, 22.
This television switch module enables the two input lines 11, 21 to be selectively switched and output to the two output lines 17, 27 by selecting four kinds of combinations using the relay switches 16, 26. For example, in the case of outputting a signal of the input line 11 to the output line 17, a high frequency input signal is output via the amplifier 12 and the relay switch 16 to the output line 17. Further, in the case of outputting a signal of the input line 21 to the output line 27, a high frequency input signal is output via the amplifier 22 and the relay switch 26 to the output line 27. In the case of outputting a signal of the input line 11 to the output line 27, a high frequency input signal is output via the amplifier 12 and the relay switch 26 to the output line 27. Further, in the case of outputting a signal of the input line 21 to the output line 17, a high frequency input signal is output via the amplifier 22 and the relay switch 16 to the output line 17. For these selected paths, high frequency signals are input to both amplifier 12 and amplifier 22.
On the other hand, in the case of outputting a signal of the input line 11 or the input line 21 to the two output lines 17, 27, a high frequency signal input from the input line 11 reaches the output lines 17 and 27 via the amplifier 13 and the relay switches 16 and 26, while a high frequency signal input from the input line 21 reaches the output lines 17 and 27 via the amplifier 23 and the relay switches 16 and 26. That is, in the case of outputting the input line 11 to the output lines 17 and 27, a high frequency signal is input to the amplifier 12, but no high frequency signal is input to the amplifier 22. Similarly, in the case of outputting the input line 21 to the output lines 17 and 27, a high frequency signal is input to the amplifier 22, but no high frequency signal is input to the amplifier 12.
As described above, in the case of providing two outputs from one input, power consumption can be reduced by operating only the amplifier to which a high frequency signal is input, namely by providing power supply only to the amplifier to which a high frequency signal is input.
However, in conventional configurations, power supply is always provided to both amplifiers, which means that they consume power more than necessary. Further, for current stabilization of an amplifying transistor, the conventional configurations generally use a method based on a resistance feedback circuit, which is insufficient in the current stability against e.g. variations or changes of the direct current amplification factor of the amplifying transistor. For this reason, the current of the amplifying transistor in some of them becomes larger than necessary, which has also been a cause of power consumption more than necessary.
Here, a specific example of the amplifiers 12, 22 is shown in FIG. 12. This amplifier circuit is an amplifier circuit using a transistor which is called a self-bias circuit. Since the current consumption of the amplifier circuit varies due to the variation of the current amplification factor hfe of the transistor, the self-bias circuit is devised so as to reduce its influence as much as possible.
The operation principle of this amplifier circuit will be described. The present circuit operates in a manner that when the collector current Ic increases, the voltage drop occurring across the resistance RL increases, which causes the base current IB flowing through the resistance RB to decrease, which causes the collector current Ic to decrease. Thus, the current consumption of the amplifier circuit can be expressed by the following equation:Ic+IB=(hfe+1)·(Vcc−VBE)/(RL+RB+hfe·RL)
From this equation, it can be seen that it is possible to reduce the influence due to the variation of hfe by setting the resistance RL to be high, and the resistance RB to be low. However, actually, if the resistance RL is made high and the RB is made low to an extent to make negligible the current variation due to the variation of hfe, the voltage drop by the resistance RL becomes very large, which results in the use of the transistor in a very low range of the collector voltage, thereby preventing a dynamic range from being ensured. Thus, conventionally, there has been no other option than to allow the current variation to some extent in the operation.
Besides, it is known to add a current mirror circuit to an amplifier circuit so as to stabilize the current consumption even if hfe varies (refer to e.g. Japanese Laid-open Patent Publication Hei 10-70419). This circuit is configured to use a current mirror circuit to provide a bias current of a signal amplifying transistor. In the case of using the current mirror circuit, it is necessary to adjust the performances of a high frequency transistor and a bias transistor, particularly the base-emitter voltages. In order to achieve this, it is necessary to integrate all these transistors at close positions on a semiconductor. This is not a problem for an integrated circuit, but is not suitable for configuring a discrete circuit. Thus, there has been a demand to stabilize current consumption in a discrete circuit configuration with high design freedom.